The suggestion is simple: Make capitals be racially neutral.
My reasoning behind this is as follows:
In the current setup and depending on where your capital is and where your opponent's capital is, you will have an additional weight to wanting to support those races. Most relevant in single-player and the fact that no capital is in an imp territory.
So I propose that the capitals be racially neutral, or perhaps your popularity in a capital built as an average of your popularity with the 5 races. To offset the effects of this proposed change, I propose that the defending party have a small defender's bonus of 50 or 100 supply cap.
This is the gist of my idea. Please share your thoughts on the matter.